Polishing grindstone and method for producing same

ABSTRACT

A polishing grindstone according to the present invention comprises abrasive grains dispersed in, bound to and held by a particulate matrix composed of rubber particles. With the polishing grindstone, the abrasive grains decreased in polishing properties owing to wear associated with polishing become easily detached from the surface of the grindstone, and fresh abrasive grains not worn are exposed at the surface, thus permitting polishing at a constantly stable speed, and ensuring stable polishing for a long term.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a polishing grindstone and a method for producing it. More particularly, the invention relates to a polishing grindstone which can perform polishing stably for a long term, and a method for producing the polishing grindstone.

2. Description of the Related Art

In recent years, circuit substrates, etc. for use in various electronic instruments have been required to have high surface smoothness, since fine wiring is formed at a high density. High surface smoothness has also been required of glass substrates, etc. for use in hard disks (magnetic disks) of computers. Such high surface smoothness is achieved by mirror finishing using an abrasive material.

Mirror finishing is generally performed by lapping the substrate to impart a certain degree of smoothness to the surface of the substrate, and then rubbing a grindstone against the surface of the substrate, while supplying cooling water, to polish the surface of the substrate.

As the above grindstone, Patent Documents 1 and 2 propose grindstones each of a structure having abrasive grains dispersed in, and fixed to, a binder such as a urethane resin or a phenolic resin. Patent Document 3 proposes a grindstone of a structure using rubber, such as silicone rubber, as a binder, and having abrasive grains dispersed in, and fixed to, the rubber.

-   -   Patent Document 1; Japanese Unexamined Patent Publication No.         2001-260037     -   Patent Document 2; Japanese Unexamined Patent Publication No.         2004-261942     -   Patent Document 3; Japanese Unexamined Patent Publication No.         2002-261054

With each of the grindstones proposed in the above Patent Documents 1 and 2, however, the abrasive grains are firmly fixed in the binder which is a rigid resin. Thus, even when the abrasive grains wear and shrink during polishing, the worn-out abrasive grains minimally come off, and fresh abrasive grains, which have not worn, are not exposed at the surface of the grindstone. As a result, the problem arises that the speed of polishing lowers in a short time, and polishing cannot be carried out stably. Thus, a dressing operation for the grindstone has to be performed where necessary. Alternatively, there is no choice but to adopt a means, such as chemical mechanical polishing in which chemical polishing using a chemical solution, e.g., an alkali solution, takes place in addition to polishing with the grindstone.

The grindstone proposed in the above Patent Document 3 has the above abrasive grains dispersed in the rubber having elasticity. Compared with the grindstones proposed in the Patent Documents 1 and 2, therefore, this grindstone permits the abrasive grains, which have been worn by polishing, to become detached, and allows fresh abrasive grains to be easily exposed, showing some improvement on the problem of the decrease in the polishing speed. Even with the grindstone of Patent Document 3, however, the abrasive grains are firmly bound to and held by the rubber. Hence, the detachment of the worn-out abrasive grains and the exposure of fresh abrasive grains fail to fully take place, thereby causing a drop in the polishing speed in a short time. Improvement on this shortcoming is sought.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide a grindstone in which abrasive grains decreased in polishing properties owing to wear associated with polishing become easily detached from the surface of the grindstone, and fresh abrasive grains not worn are exposed at the surface, thus permitting polishing at a constantly stable speed, and ensuring stable polishing for a long term; and a method for producing such a grindstone.

The inventor diligently conducted studies on a grindstone of a structure having abrasive grains dispersed in, and fixed to, a binder. These studies led to the discovery that the aforementioned problems could be solved effectively by using rubber particles as a binder, which is to fix abrasive grains, and dispersing abrasive grains in, and fixing them to, a particulate matrix formed from such rubber particles. Based on this finding, the inventor accomplished the present invention.

According to the present invention, there is provided a polishing grindstone comprising abrasive grains dispersed in, bound to and held by a particulate matrix composed of rubber particles.

In the polishing grindstone of the present invention, it is preferred that

(1) the particle size of the rubber particles be within the range of 5 to 300 μm, and

(2) the abrasive grains be incorporated in an amount of 40 to 80 parts by weight per 100 parts by weight of the rubber particles.

According to the present invention, there is also provided a method for producing a polishing grindstone, comprising:

a mixing step of mixing rubber particles and abrasive grains to prepare a particulate mixture;

a compression molding step of compression molding the particulate mixture into a predetermined shape; and

a heat treatment step of heat-treating a molded product obtained in the compression molding step.

In the method for production according to the present invention, it is preferred that

(3) the abrasive grains be used in an amount of 40 to 80 parts by weight per 100 parts by weight of the rubber particles,

(4) the compression molding be performed at a pressure of 50 to 300, kg/cm²,

(5) the heat treatment be performed at a temperature of 150 to 250° C., and

(6) the compression molding and the heat treatment be performed simultaneously.

In the polishing grindstone of the present invention, the abrasive grains are dispersed in, and bound to, the particulate matrix formed from the rubber particles. Thus, the abrasive grains worn by polishing and present on the surface of the polishing grindstone become easily detached from the surface, and fresh abrasive grains which have not been worn are exposed at the surface of the polishing grindstone. As a result, the disadvantage that the polishing speed decreases in a short time is effectively avoided, and polishing can be performed stably for a long term. Furthermore, polishing can be effectively carried out only mechanically using this grindstone, without combination with chemical polishing using a chemical solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the sectional structure of a polishing grindstone according to the present invention.

FIG. 2 is a schematic view showing the sectional structure of a polishing grindstone which has been publicly known.

FIG. 3 is a perspective view showing a polishing wheel comprising the polishing grindstone of the present invention mounted on a base plate.

FIG. 4 is a graph showing the relationship among the number of glass substrates polished, the value of a current flowing through a rotating shaft of the polishing wheel, and the amount of polishing of the glass substrate when the glass substrate was polished using the polishing wheel of the present invention prepared in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

Polishing Grindstone

With reference to FIG. 1, the polishing grindstone of the present invention comprises a particulate matrix 3 formed from rubber particles 1, and abrasive grains 5 dispersed in and fixed to the matrix 3. That is, the rubber particles 1 retain a particle shape, and are fusion bonded to each other on the surfaces of the respective particles, thereby forming the particulate matrix 3. The rubber particles 1 are also fusion bonded to the surfaces of the abrasive grains 5 dispersed in the matrix 3, whereby the abrasive grains 5 are fixed in the matrix 3. As shown in FIG. 1, abrasive grains 5 a are exposed at the surface of the polishing grindstone. By rubbing these abrasive grains 5 a against the surface of a predetermined substrate, the surface of the substrate is polished.

FIG. 2 shows the sectional structure of a polishing grindstone which has been publicly known. This polishing grindstone has a structure in which abrasive grains 5 are dispersed and fixed in a matrix 7. This matrix 7 is formed from various resins or rubber. A major difference of this polishing grindstone from the polishing grindstone of the present invention is that the matrix 7 is completely integrated by curing the resin or rubber, and the abrasive grains 5 are dispersed and fixed in the matrix 7 of the resin or rubber thus integrated.

That is, with the publicly known polishing grindstone as shown in FIG. 2, the abrasive grains 5 are embedded in the matrix and firmly fixed thereto (namely, the bonding strength between the matrix 7 and the abrasive grains 5 is high). Thus, even when the abrasive grains 5 exposed at the surface are worn and decreased in size by polishing, the exposed abrasive grains 5 minimally become detached from the surface of the grindstone, and fresh abrasive grains 5 which have not worn are scarcely exposed at the surface. As a result, when polishing is carried out, the polishing speed lowers in a short time.

With the polishing grindstone of the present invention shown in FIG. 1, on the other hand, the matrix 3 is formed by the fusion bonding of many rubber particles 1 at the surface. The abrasive grains 5 are present in the gaps between the rubber particles 1, and the abrasive grains 5 are fixed by their surface fusion to the rubber particles 1. In the polishing grindstone of FIG. 1, as compared with the polishing grindstone of FIG. 2, the area of contact between the abrasive grains 5 and the matrix 3 (rubber particles 1) is so small that the bonding force working between them is low. Thus, the abrasive grains 5 present on the surface and worn by polishing become easily detached. When the rubber particles 1 existent on the surface wear or come off, fresh abrasive grains 5 which have not worn are promptly exposed at the surface of the grindstone. Thus, polishing can be performed stably for a long term, without a decrease in the polishing speed in a short time.

In the polishing grindstone of the present invention, the particle size of the rubber particles 1 constituting the particulate matrix 3 is preferably in the range of 5 to 300 μm. If the particle size of the rubber particles 1 is too great, the holding power for the abrasive grains 5 (i.e., the bonding force acting between the matrix 3 and the abrasive grains) declines to a lower level than required. During polishing, therefore, the abrasive grains 5 become detached more easily than necessary, thereby potentially deteriorating polishing properties. If the particle size of the rubber particles 1 is excessively small, the holding power for the abrasive grains 5 becomes so high that the worn-out abrasive grains 5 a present on the surface of the grindstone are detached with difficulty. As a result, the decrease in the polishing speed is apt to occur.

To retain the bonding force between the rubber particles 1 and the abrasive grains 5 moderately, the rubber particles 1 are preferably cured rubber. For example, their rubber hardness by a durometer (JIS K6253) is preferably in the range of 50 to 80. If the hardness of the rubber particles 1 is lower than this range, the area of contact between the abrasive grains 5 and the rubber particles 1 becomes large during a compression molding step to be described later, so that the bonding force on the abrasive grains 5 increases to a higher level than required. Consequently, the worn-out abrasive grains 5 a have difficulty in falling from the surface, leading to a possibility for a decrease in the polishing speed. If the hardness of the rubber particles 1 is higher than the above range, the surface fusion of the rubber particles 1 to each other, or the surface fusion of the rubber particles 1 to the abrasive grains 5 becomes difficult. A resultant decline in the holding power for the abrasive grains 5 may deteriorate the polishing properties, or in some cases, make it difficult to retain the shape of the grindstone.

Furthermore, the rubber forming the above-mentioned rubber particles 1 is not limited. For example, suitable rubber materials can be used, such as acrylonitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), polyurethane rubber, polyurea rubber, urethane-urea rubber, acrylic rubber, butadiene rubber, isoprene rubber, silicone rubber, fluororubber, and natural rubber. Of these rubbers, NBR and SBR are preferred in that they facilitate the formation of the particulate matrix 3 shown in FIG. 1 upon heat fusion of the particle surfaces by compression molding and heat treatment to be described later.

In the present invention, abrasive grains publicly known per se are used as the abrasive grains 5 dispersed and fixed in the particulate matrix 3 composed of the rubber particles 1. Their examples are rigid inorganic powders, typified by cerium oxide, silicon oxide (quartz, fused silica, etc.), alumina, silicon carbide, and zirconium oxide. Such abrasive grains 5 are preferably a fine powder from the point of view of high polishing properties. Generally, its grain size is preferably in the range of 20 μm or less.

Generally, it is preferred that the abrasive grains 5 be dispersed in an amount of 40 to 80 parts by weight per 100 parts by weight of the rubber particles 1, although the amount differs according to the types of the abrasive grains 5 and the rubber particles 1. This amount is preferred in ensuring excellent polishing properties for a long term.

The above-mentioned polishing grindstone of the present invention is formed in the shape of a ring, and adhered and fixed onto a metallic support base plate 14 of a similarly ring shape by an adhesive or the like, for use as a polishing wheel 10, as shown in FIG. 3 (in FIG. 3, the polishing grindstone is indicated at 12). That is, the polishing wheel 10 has the surface of the polishing grindstone 12 as a flat polishing surface 12 a, and has a through-hole 12 b formed in its central portion. While the polishing wheel 10 is rotated and a polishing fluid (cooling water or a grinding fluid) is being supplied, the polishing surface 12 a of the polishing grindstone 12 during rotation is pressed against the surface of a predetermined substrate to perform polishing. On this occasion, a chemical solution such as an alkali solution may be supplied, if necessary, to perform chemical polishing concurrently with mechanical polishing. According to the polishing grindstone 12 of the present invention, excellent polishing properties can be maintained for a long term. Thus, polishing can be performed by mechanical polishing alone, without combination with such chemical polishing. This is a great advantage of the present invention.

Production of Polishing Grindstone

The above-described polishing grindstone is produced by a mixing step, a compression molding step, and a heat treatment step.

—Mixing Step—

In the mixing step, the rubber particles comprising the rubber and the abrasive grains are mixed to prepare a particulate mixture. That is, in forming the particulate mixture, melt integration such as melt kneading is not performed. Instead, mixing of the rubber particles and the abrasive grains is carried out by dry mixing using a mixer such as a Henschel mixer.

Preferably, the rubber particles are a powder of a cured rubber, and have an average particle size in the aforementioned range. Such a powder of the cured rubber can be easily prepared, for example, by suspension polymerization or emulsion polymerization using a polymerizable composition containing a curing agent (e.g., an organic peroxide) or a vulcanizing agent such as sulfur in such an amount as to impart predetermined hardness, and, in addition to a monomer component for rubber formation, and a polymerization initiator. The average particle size of the cured rubber powder can be adjusted, as appropriate, by the amount of a surface active agent and stirring conditions which are used during suspension polymerization, etc.

The amounts of the rubber particles and the abrasive grains should desirably be set such that the amount of the abrasive grains is in the range of 40 to 80 parts by weight per 100 parts by weight of the rubber particles, as stated earlier.

—Compression Molding Step—

Then, the particulate mixture prepared in the above manner is charged into a predetermined mold, and compression molded into a predetermined shape, such as an annular shape, as shown, for example, in FIG. 3. The pressure during this molding varies according to the type, hardness, etc. of the rubber particles, but usually, is of the order of 50 to 300 kg/cm². By this compression step, the rubber particles and the abrasive grains are brought into intimate contact giving a moderate contact area. If this pressure is higher than required, the contact area between the rubber particles and the abrasive grains is excessively large, or the force of intimate contact between the abrasive grains and the rubber particles or the force of intimate contact between the rubber particles is too high. As a result, subsequent heat treatment to be described later results in too high a bonding force working between the rubber particles (particulate matrix) and the abrasive grains. This poses difficulty in the detachment of the abrasive grains worn by polishing, thereby causing decline in the polishing properties in a short time. If this pressure is too low, on the other hand, the holding power for the abrasive grains lowers, or the binding force on the rubber particles relative to each other decreases, making it difficult to ensure adequate polishing properties.

The compression molding step presses the rubber particles to a somewhat flat shape, but the particle size of the flattened rubber particles is substantially the same as the particle size of the rubber particles used as the starting material.

—Heat Treatment Step—

In this step, the molded product obtained by the above-described compression molding step is heated to perform the surface fusion of the rubber particles to each other and the surface fusion of the rubber particles to the abrasive particles, with the particle shape of the rubber particles being maintained. Thus, the temperature of the heat treatment varies according to the type and hardness of the rubber particles used. Generally, however, the preferred heat treatment temperature is of the order of 150 to 250° C. If this heat treatment temperature is too high, the particle shape cannot be maintained, so that the rubber particles may be deformed and firmly integrated like a sea. If the heat treatment temperature is lower than necessary, the surface fusion of the rubber particles to each other and the surface fusion of the rubber particles to the abrasive particles fail to take place. Thus, a decrease in the strength of the molded product itself occurs, making it difficult to obtain a grindstone satisfactory in polishing properties.

The heat treatment time usually may be 1 to 5 hours.

In the present invention, the above-described heat treatment can be performed after compression molding. However, it is preferred that the heat treatment be performed under pressure simultaneously with compression molding in order to carry out the surface fusion of the rubber particles to each other and the surface fusion of the rubber particles to the abrasive particles in a short time and effectively.

The polishing grindstone of the present invention, produced in the above manner, is adhered and fixed to a predetermined support base plate, as shown, for example, in FIG. 3, for use as a polishing wheel for polishing of various substrates.

EXAMPLES Example 1

The following materials were used as a rubber powder and abrasive grains:

Rubber Powder

Material: NBR cured rubber

Particle size: 5 to 300 μm

Rubber hardness (JIS K6253): 50 (by a durometer)

Abrasive Grains

Material: Cerium oxide

Grain size: 0.1 to 10 μm

The above rubber powder (100 parts by weight) and 50 parts by weight of the abrasive grains were dry-mixed to prepare a particulate mixture.

The resulting particulate mixture was compression molded into the shape of a ring at a pressure of 100 kg/cm², and then held at 150° C. for minutes, with this pressure being kept, to carry out heat treatment, thereby producing a ring-shaped grindstone of the following dimensions:

Outer diameter: 300 mm

Inner diameter: 100 mm

Thickness: 10 mm

Microscopic observation of a section of the above grindstone showed that the cured rubber particles were surface-fused together, with their particle shapes being retained, to form a particulate matrix, and that the abrasive grains were fixed in the particulate matrix by surface fusion, as indicated in FIG. 1.

The grindstone obtained above was adhered and fixed to a ring-shaped metallic support base plate as shown in FIG. 3. Using the resulting polishing wheel, the surface of a disk-shaped borosilicate glass substrate fulfilling the following specifications was polished under the following conditions by rotating the grindstone while superposing a peripheral edge portion of the grindstone on a peripheral edge portion of the glass substrate:

Borosilicate Glass Substrate:

-   -   Diameter: 150 mm     -   Maximum surface roughness R_(y) (JIS B0601-1994): 0.1925 μm         Polishing Conditions:     -   Polishing pressure (pressing force of grindstone on glass         substrate): 150 g/cm²     -   Rotational speed of grindstone: 30 m/sec (peripheral edge         portion)     -   Polishing fluid: Water (amount supplied: 2 L/min)     -   Polishing time: 150 seconds

Upon the above polishing, the maximum surface roughness R_(y) of the glass substrate surface became 0.0335 μm, meaning that a smooth mirror surface was obtained. This finding shows that with the above grindstone, the worn-out abrasive grains on the surface of the grindstone are detached, and fresh abrasive grains are sequentially exposed at the surface, whereby polishing is performed effectively. Thus, it has been demonstrated that a mirror surface with high smoothness can be obtained, without the need to perform chemical polishing using an alkali solution.

Under the above-mentioned conditions, 10 of the glass substrates were polished consecutively. FIG. 4 shows the relationship among the number of the glass substrates polished, the value of a current flowing through the rotating shaft of the polishing wheel, and the amount of polishing per glass substrate. The value of the current flowing through the rotating shaft of the polishing wheel represents load imposed on the rotating shaft during polishing.

The results of FIG. 4 show that after 2 to 4 of the glass substrates are polished, the amount of polishing and the value of the current are stabilized. This finding shows that upon polishing of the glass substrate, fresh abrasive grains are sequentially exposed, and polishing takes place stably with a constant efficiency, thus obviating the necessity for dressing of the grindstone, for example. 

1. A polishing grindstone comprising abrasive grains dispersed in, bound to and held by a particulate matrix composed of rubber particles.
 2. The polishing grindstone according to claim 1, wherein a particle size of the rubber particles is within a range of 5 to 300 μm.
 3. The polishing grindstone according to claim 1, wherein the abrasive grains are incorporated in an amount of 40 to 80 parts by weight per 100 parts by weight of the rubber particles forming the particulate matrix.
 4. A method for producing a polishing grindstone, comprising: a mixing step of mixing rubber particles and abrasive grains to prepare a particulate mixture; a compression molding step of compression molding the particulate mixture into a predetermined shape; and a heat treatment step of heat-treating a molded product obtained in the compression molding step.
 5. The method according to claim 4, wherein the abrasive grains are used in an amount of 40 to 80 parts by weight per 100 parts by weight of the rubber particles.
 6. The method according to claim 4, wherein the compression molding is performed at a pressure of 50 to 300 kg/cm².
 7. The method according to claim 4, wherein the heat treatment is performed at a temperature of 150 to 250° C.
 8. The method according to claim 4, wherein the compression molding and the heat treatment are performed simultaneously. 